Shunhu Yang

Zircon ages and geochemical data of the Biluutiin ovoo ophiolite: implication for the tectonic evolution of South Mongolia

The early Palaeozoic tectonic evolution of South Mongolia is not well constrained due to the limited exposure of early Palaeozoic rocks in the area and the scarcity of both geochemical and geochronological data. In order to help rectify this situation, we have conducted detailed studies on the Biluutiin ovoo ophiolite in South Mongolia to provide constraints on the tectonic evolution of the region during this period. The Biluutiin ovoo ophiolite consists of ultramafic rocks, mylonitic gabbro, basalt, tuff sandstone, plagiogranite, calcite, and chert. Gabbro and plagiogranite samples from the ophiolitic complex yielded SHRIMP zircon ages of 525 ± 5 Ma and 503 ± 6 Ma, respectively. Biluutiin ovoo ophiolitic basalts display LREE and LILE enrichment and strong HFSE depletion, indicating that the ophiolite is supra-subduction zone (SSZ) type. Plagiogranite with adakite-like geochemical compositions suggests that palaeo-ocean subduction occurred in South Mongolia during Cambrian time. Intruding granite yielded a SHRIMP zircon age of 353 ± 2 Ma, indicating that the ophiolite was emplaced before early Carboniferous time. Identification of the Cambrian ophiolitic complex and the occurrence of Cambrian adakites indicate that southern Mongolia underwent a period of active volcanism during the Cambrian. The Cambrian formations are likely correlated to the early Palaeozoic subduction-accretion belt of Western Mongolia.

Genesis and evolution of subduction-zone andesites: evidence from melt inclusions

Andesite magmatism plays a major role in continental crustal growth, but its subduction-zone origin and evolution is still a hotly debated topic. Compared with whole-rock analyses, melt inclusions (MIs) can provide important direct information on the processes of magma evolution. In this article, we synthesize data for melt inclusions hosted by phenocrysts in andesites, extracted from the GEOROC global compilation. These data show that melt inclusions entrapped by different phenocrysts have distinct compositions: olivine-hosted melt inclusions have basalt and basaltic andesite compositions, whereas melt inclusions in clinopyroxene and othopyroxene are mainly dacitic to rhyolitic. Hornblende-hosted melt inclusions have rhyolite composition. The compositions of melt inclusions entrapped by plagioclase are scattered, spanning from andesite to rhyolite. On the basis of the compositional data, we propose a mixing model for the genesis of the andesite, and a two-chamber mechanism to account for the evolution of the andesite. First, andesite melt is generated in the lower chamber by mixing of a basaltic melt derived from the mantle and emplaced in the lower crust with a felsic melt resulting from partial melting of crustal rocks. Olivine and minor plagioclase likely crystallize in the lower magma chamber. Secondly, the andesite melt ascends into the upper chamber where other phenocrysts crystallize. According to SiO2-MgO diagrams of the MIs, evolution of the andesite in the upper chamber can be subdivided into two distinct stages. The early stage (I) is characterized by a phenocrystal assemblage of clinopyroxene + othopyroxene + plagioclase, whereas the late stage (II) is dominated by crystallization of plagioclase + hornblende.